Double check yourself, before you double wreck yourself. Make sure everything you send to a company, whether a résumé, an email or a portfolio, is good to go. Double check your grammar and wording, and for God’s sake use spell check! This is especially important when it comes to the company’s name. Don’t spell their name wrong and be sure to type it how they type it (e.g. Problogger, not Pro Blogger).
You can pick writing jobs directly from a live job board or have to bid on a job and hope that the job is awarded to you by the relevant client, depending on how the site works. Some sites may require you to take an English grammar and writing test and some may require you to submit a sample or samples of your work while others may require you to take a grammar test to evaluate your skills. If you pass any relevant test, the relevant site will let you take on writing jobs.